Paint spraying method and apparatus

ABSTRACT

A method and apparatus for spraying paint and similar coatings. In one form the paint is sprayed at low pressure from a flat fan nozzle. A flat fan of air impinges upon the fan of paint at an angle of from 30*-70*. The air atomizes the paint to form a soft spray with low-forward velocity. In another form of the method, two fans of paint are projected toward one another at an angle and an air fan is directed along the bisector toward the zone of intersection. The air fan atomizes both streams of paint. An electrode can be placed in the air stream to ionize the air stream which in turn charges the paint particles. The width of the paint spray pattern is varied by varying the included angle of the air fan. A spray gun is disclosed having two tubular paint nozzles for projecting two fan-shaped fans of paint. An air nozzle is mounted between the paint nozzles for projecting a fan of air against the paint fans to atomize the paint. The air nozzle is adjustable to provide air fans of different angles for varying the width of the paint spray pattern. An electrode is disposed in the air nozzle for ionizing the air stream. This electrode is energized through a resistor and flexible cable. The cable is spring-urged toward the resistor so that when the resistor is removed, the cable shifts causing a safety switch to open deenergizing the gun. When a nozzle mounting member is removed, the cable and surrounding tube are shifted forwardly to automatically close a valve to seal off the air and electrical conduits. Removable caps are provided on the ends of the tubular paint nozzles for facilitating cleaning of the nozzles.

Waited atet 3,635,400 Nerd et a1. 5] Jun. 1%, 1972 541 PAINT SPRAYINGMETHOD AND [57 ABSTRACT APPARATUS Inventors: Eric T. Nor-d, Oberlin;Samuel R. Rosen,

Lorain; Don R. Scarbrough, E1yria; Burton J. Vllngl, Amherst, all ofOhio; PeterW. Runstadier, JB, Hanover, NH.

[73] Assignee: Nordson Corporation, Amherst, Ohio [22] Filed: Mar. 27,1970 [21] Appl. No.: 23,227

[52] [1.5. CI. ..239/15, 239/422 [51] lint. Cl. ...B05b 5/00, F23d 11/28[58] Field ofSearch.... .....239/422, 3,15, 543, 544

[56] References Cited UNITED STATES PATENTS 1,881,345 10/1932 Beatty eta1 ..239/422 3,049,092 8/1962 Sedlacsik et al. 239/15 X 2,302,28911/1942 Cook 239/15 X 3,067,949 12/1962 Sigvardsson et a1. ..239/l53,169,882 2/1965 Juvinall et a1 ..239/3 X 3,540,653 11/1970 Fabre..239/3 X Primary Examiner-Lloyd L. King AttorneyWood, Herron & Evans Amethod and apparatus for spraying paint and similar coatings. in oneform the paint is sprayed at low pressure from a flat fan nozzle. A flatfan of air impinges upon the fan of paint at an angle of from 30-70. Theair atomizes the paint to form a soft spray with low-forward velocity.In another form of the method, two fans of paint are projected towardone another at an angle and an air fan is directed along the bisectortoward the zone of intersection. The air fan atomizes both streams ofpaint. An electrode can be placed in the air stream to ionize the airstream which in turn charges the paint particles. The width of the paintspray pattern is varied by varying the included angle of the air fan.

A spray gun is disclosed having two tubular paint nozzles for projectingtwo fan-shaped fans of paint. An air nozzle is mounted between the paintnozzles for projecting a fan of air against the paint fans to atomizethe paint. The air nozzle is adjustable to provide air fans of difierentangles for varying the width of the paint spray pattern. An electrode isdisposed in the air nozzle for ionizing the air stream. This electrodeis energized through a resistor and flexible cable. The cable isspring-urged toward the resistor so that when the resistor is removed,the cable shifts causing a safety switch to open deenergizing the gun.When a nozzle mounting member is removed, the cable and surrounding tubeare shifted forwardly to automatically close a valve to seal off the airand electrical conduits. Removable caps are provided on the ends of thetubular paint nozzles for facilitating cleaning of the nozzles.

20 Claims, 19 Drawing Figures PATENTED M 1912 7 "3.635 400 SHEET 1 OF 717/153 r700 flff PATENTEDJAN18I972 3, 35

SHEET 3 OF 7 I NVENTOIL PATEMTED JAM 8 I372 SHEET 5 UF 7 PAINT SPRAYKNGMETHOD AND APPTUS BACKGROUND OF THE INVENTION This invention relates tomethods and apparatus of spraying paints, lacquers and similar coatingmaterials and is particularly directed to a novel method and apparatusfor effecting air atomization of the paint and for electrostaticallycharging the paint if desired so that the electrostatic field forces canbe utilized to increase the effectiveness of paint deposition.

In the past there have evolved two distinct types of paint sprayingequipment. The first type can be characterized as airiess" sprayingequipment. In an airless type of spraying apparatus, a paint stream isforced through an orifice under a relatively high pressure, for example,a pressure of the order of BOO-1,000 pounds. As the paint is propelledthrough the small orifice it is broken up, or atomized, into very finedroplets. The paint spray formed from the gun moves at a relatively lowvelocity toward the article to be painted. In many cases the paintspraying operation is carried out in the presence of a high-voltageelectrostatic field in which the work to be coated is kept at, or closeto, ground potential while the atomized paint particles are charged to arelatively high potential. These particles are then urged toward thework by the forces of the electrostatic field.

This type of system has the recognized advantage of providing a veryhigh deposition efficiency, i.e., a large portion of the paint sprayemitted from the gun is effectively deposited on the article to becoated. On the other hand, in some installations the airless spray-typeapparatus has certain inherent shortcomings. One such objectionablecharacteristic is that the system requires a paint supply systemoperated at a relatively high pressure. As a result, it is not feasibleto disconnect a gun from one high-pressure paint line and quicklyconnect it to another high-pressure paint line when, for example, it isdesired to change the color of coating being sprayed.

The second general type of spray equipment does not require ahigh-pressure paint source. This second type of spray system, which isknown as air spray equipment, relies upon a stream of air to break upthe paint into particle size suitable for spraying. In conventional airspray equipment, the paint is extruded from a nozzle in a generallyrodlike form and is subjected to a high-pressure blast of air. In atypical air spray installation, the air pressure at the gun is under apressure of approximately the order of 75 pounds per square inch. Theair is used in large quantifies; for example, it is common practice toutilize an airflow of 14 or 15 standard cubic feet of air per minute tocarry out atomization.

Air spray systems of this type do present certain advantages in thatthey can be utilized to atomize particularly difficult types of paintand can more readily be provided with quick disconnect couplings to thepaint line since the paint line is maintained at a relatively lowpressure of, for example, 50 psi. At the same time, however, prior artpaint spray guns are subject to several disadvantages.

In the first place, the large quantity of high-velocity air used toatomize the paint together with the airflow it induces causes asubstantial portion of the paint spray to be carried past the workpieceand wasted even when an electrostatic charge is applied to the paint.Moreover, the rapidly moving air has a high kinetic energy which causesit to bounce back or rebound from the surface being coated carrying withit entrained paint particles. Consequently, an appreciable portion ofthe paint which is directed toward the workpiece is wasted due torebound.

Another inherent disadvantage of conventional air spray equipment is theproblem of ventilation. Specifically, because of the high volume of airemitted by each spray gun, and in many installations there are severalguns operating simultaneously, a ventilation system must be providedhaving a high capacity'for capture of paint particles entrained in thelarge mass of high-speed air, for example, capture velocities of 150feet per minute.

Also, because of paint entrained in the large amount of overspray andrebound, water curtains or other types of filters must be provided forpreventing sizable quantities of paint from escaping and polluting theatmosphere.

SUMMARY OF THE INVENTION The principal object of the present inventionis to provide a novel method and apparatus for spraying paint inwhichrelatively small quantities of air at a relatively low pressure areutilized to atomize the paint in a very effective manner so that theresultant paint spray is a soft, finely divided spray not unlike thatwhich could heretofore be produced only by an airless gun.

At the same time, the present method and apparatus are effective toretain the advantages inherent in any air-type of spray gun, i.e., theability to atomize various difiicult types of coating material and thefacile interconnectability to various paint lines so that the spray guncan be converted quickly from spraying one color or type of coating toanother.

More particularly, the present invenu'on is predicated in part upon theconcept of spraying paint by emitting a thin, flat fan-shaped stream ofpaint from one nozzle and a fan-shaped stream of air from a secondnozzle. The paint fan in most cases is initially in the form of acontinuous sheet. However, the fan can be in the form of a thin,substantially planar discontinuous spray; for example, one which isalready partially atomized. The fan-shaped stream of paint and airimpinge upon one another at a substantial angle of from approximately3070. The air thus exerts an optimum shearing force upon the paint fanand breaks the paint down into small particles of a low mean particlesize with substantially no particles of an objectionably large size.

This invention is further predicated in part upon the empiricaldiscovery and determination of various physical relationships and valuesof certain parameters which make it possible to obtain an atomizationsuitable for fine finishing purposes utilizing as one starting componenta low-pressure fan of paint. A flat stream of paint from any presentlyknown form of fan nozzle operated at low pressure is inherently in avery difficult form to atomize into small particles with the uniformityrequired for producing satisfactory painted surfaces.

The difficulties involved are due to the fact that the paint stream isnot a uniformly thin flat sheet. Rather, the sheet in cross section issomewhat like a dumbbell with a thin center web and two enlargedportions, one at each edge. These enlarged portions are in factlongitudinal beads or streamers." These two streamers are several timesthe thickness of the central web portion and tend to remain integral andresist atomization even when the rest of the sheet is broken up intoparticles. It will readily be appreciated that no matter how finely mostof the paint is atomized, if the streamers are not broken up intoparticles of generally the same fineness, the quality of the appliedcoating will suffer and may well become totally unacceptable. Thepresent paint spray method is effective to cause effective breakup ofall parts of the paint stream, including the streamers, into smallparticles while using only a relatively small quantity of air.

In accordance with flie present invention, the air stream is projectedunder a relatively low pressure, for example, of the order of 9 to 45pounds per square inch at the gun, while the paint is sprayed under alow pressure of, for example from 30-80 pounds per square inch. Thismethod of atomization is so effective that the quantity of air requiredto deposit a relatively large quantity of paint, for example, 22-25fluid ounces per minute of a typical baking enamel, is approximately 7-8standard cubic feet per minute. As a result of this small volume oflow-pressure air and the fine atomization of the paint, the resultantspray is in the nature of a soft spray, or slowly moving fog, having alow forward velocity.

One of the principal advantages of this method of spraying is that itresults in a substantially higher deposition efficiency thanconventional air spraying methods. There is substantially less paintwaste due to overspray and rebound which is minimized or practicallyeliminated.

Another advantage of the present spray method is that the relativelysmall quantity of air involved and the smaller quantity of paintentrapped in the exhausted air greatly simplifies the problem ofventilation and prevention of atmospheric contamination.

Another object of the present invention is to provide a novel method forreadily changing the width of the paint spray pattern. This aspect ofthe present invention is predicated upon our determination that thewidth of the paint spray can be controlled by changing the includedangle of the air fan without in any way changing the paint fan.

In accordance with the present invention, the air fan is selectivelyemitted from one of a series of nozzle openings constructed so that thenozzle openings not only simultaneously produce air fans of difi'erentincluded angles, but also produce air fans which behave much as thoughthey are emitted from a point pressure source, the distance from thenozzle opening of which is varied in an inverse relationship to theangular size of the nozzle opening. By virtue of this relationship, theheight of the air fan along its line of impingement with the paintstream is maintained substantially constant so that all portions of thepaint stream are effectively atomized with a minimum air usage.

In addition to the method concepts disclosed above, the presentinvention also comprehends a preferred spray painting method utilizingin part the method described. More particularly, in the preferred methoda fan of air is in effect enveloped between two fans of paint. The fansof paint are directed toward each other with the air fan bisecting theangle so that it impinges upon both paint streams in substantially thesame zone. This method results in an even better atomization than thesingle spray method, apparently due to the fact that in order to escape,a major portion of the air must pass through one or the other paintstreams or through the paint particles being separated from the streams.Thus, the kinetic energy of the air is more effectively utilized toshear the paint and cause its breakup into particles of a small meanparticle size.

The present two-paint stream method of spraying is also particularlyadvantageous when employed in conjunction with the present novel methodof electrostatic charging. More particularly, in accordance with thepresent charging method, the paint is not charged directly or by anelectrode in close proximity to the paint stream. Rather, it is ourconcept to ionize the air fan prior to its impingement upon the paintstreams by passing the air fan through a corona discharge surrounding anelectrode at a high potential. As explained above, the air fan thenimpinges upon the paint streams and causes the atomization of the paintstreams.

The ions present in this air stream have a high mobility and aresubjected to the forces of the surrounding electrostatic field.Specifically, these ions, which are charged and exist in a field of highpotential, move toward the paint streams and fog of paint dropletsexisting in regions of a lower potential. The ions attach themselves tothe paint particles which are of substantially lower mobility. Thesecharged paint particles are propelled forward by the air stream andunder the additional influence of the electrostatic field forces arebrought into contact with the surface to be coated which is normallymaintained at ground potential.

We have determined that the two-paint stream method results insubstantially more effective charging of paint particles than theone-spray method and believe that this is due to the fact that the ionsare to a large degree surrounded by .the paint streams and particles anddo not have a free path to one side of the paint as is the case whenonly a single paint spray is utilized.

The present method of charging paint is not only advantageous because ofits high efficiency, but is further advantageous because the entirepaint supply system, including the tube supplying paint to the gun, thepaint pump and the paint reservoir, or tank, remain at or near groundpotential.

in contrast, in prior art electrostatic spraying systems, particularlywhen a conductive paint or coating, such as one of themetallic-containing finishes utilized in the automobile industry, isused, the entire paint system became charged to the same order ofpotential as the electrode, e.g., 75,000 volts. This necessitatedcareful insulation of the entire paint system including the paintreservoir. in the event of insulation failure, for example in the paintsupply tube, the high potential present could, and not infrequently did,result in fires, electrical shock hazards, and other operationaldifiiculties. All of these problems are completely eliminated by thepresent charging method in which no charge is applied directly to thepaint either by an electrode in contact with the paint or by anelectrode in close proximity to the paint stream.

In addition to the method aspects of the present invention, theinvention is directed to the provision of a novel spray gun for carryingout the method. More particularly, the present spray gun includes ahandle, a forwardly extending barrel and a nozzle assembly mounted atthe forward end of the barrel.

The nozzle assembly comprises two tubular paint nozzles effective todirect thin flat fan-shaped paint sheets toward the axis of the gun, andan air nozzle mounted between the paint nozzles and efi'ective to directa flat fan-shaped stream of air along the axis of the gun toward thezone of intersection of the two paint fans. In accordance with thepresent invention the air nozzle and paint nozzles are located so thatthe distance of the air nozzle from the zone of air-paint impingement isonly a fraction of the distance of the paint nozzles from the zone ofair-paint impingement. As a result, the velocity of the air issuing fromthe nozzle is attenuated only minimally prior to its impingement withthe paint fans. Each of the paint fans and air fan meet at a substantialangle of the order of 30 to 70. As a result, the air fan exerts anoptimum shearing force on the paint fans and breaks up both fanssimultaneously into a spray of particles of small mean particle sizefree from inordinately large particles.

One of the advantages of the present gun is that it is quite compact andmaneuverable. Despite the fact that the gun utilizes three separatenozzles it is as small and easy to handle as previous spray guns of theair or airless type.

It is another objective of the present invention to provide a spray gunincorporating paint charging means of a substantially increasedefficiency. In fact, the present electrostatic charging system is soefiicient that when operated at 50,000 volts or even less, it providesthe same deposition efficiency as is attained using a conventionalelectrostatic charging mechanism operating at a potential of 75,000volts.

More particularly, in accordance with the present invention the gunincludes an electrode mounted within the center air nozzle in alignmentwith the nozzle discharge opening. This electrode is effective toestablish a corona through which the air passes prior to its dischargefrom the nozzle. As the air passes through the corona the gas moleculesare ionized and dust particles charged. These charged particles moveunder the electrostatic field forces toward the paint which is at a lowpotential and are subsequently deposited on the low-mobility paintparticles.

As indicated above, one of the principal advantages of this invention isthat the paint stream itself does not become charged so that there is atmost a negligible charge buildup in any portion of the paint supplysystem.

Another objective of the present invention is to provide a chargingcircuit which is substantially safer to use than the prior art. Moreparticularly, in many conventional prior art charging systems thecharging electrode projects forwardly from the end of the spray gun.This high-potential electrode is thus in an exposed position in which itcan be touched accidentally by a workman or can be shorted by accidentalcontact with a grounded conductor. In contrast, the electrode of thepresent invention is buried within the air nozzle where it iseffectively shielded from any contact with either the person using thegun or a grounded surface.

The present gun also embodies a second important safety feature whichautomatically prevents the application of a high potential to the gun inthe event that the current-limiting safety resistor is left out. Moreparticularly, it is common practice to provide a resistor in series withthe electrode and the cable connecting the power pack to the gun. When aspray gun is overhauled, the resistor is often removed and at times aworkman may neglect to replace it when reassembling the gun. When such aresistor is omitted the full potential from the power pack can becapacitively discharged across the void and applied to the gunelectrode. In such a case, if a gun is brought too close to a groundedarticle, a spark may occur which might cause an ignition of the paintmaterial or cause a painful electrical shock to the operator.

In accordance with the present invention, this is prevented by a novelsafety switch arrangement in which the main power is automatically cutoff whenever the resistor is removed from the gun.

Another objective of the present invention is to provide a constructionin which the air and electrical conduits are automatically sealed offwhen the gun is disassembled by removing the nozzle assembly. Inaccordance with the present invention, this is accomplished by providinga valve seat at the forward end of the air and electrical conduit in thegun barrel and by providing a shiftable sleeve having a valve-closingplug at its forward end. When the gun is disassembled, this sleeve isspring-urged to the valve closing position to seal off the entrance tothe air and electrical conduit. In so moving, it also cuts off the powersupply.

A still further objective of the present invention is to provide a novelform of air nozzle for selectively varying the air spray to change thesize of the paint spray pattern.

In a preferred form of nozzle, the angle of air spray is changed byselectively emitting the air through one of a series of openings in anozzle plug. The plug is provided with internal arcuate surfaces ofdifferent radiiadjacent to each nozzle opening. We have made theempirical discovery and determination that air projected from such anozzle appears to have somewhat the same properties as air emitted froma point source which is displaced from the nozzle opening a distancecorrelated with the radius of the internal nozzle surface.

To obtain a fan spray having a smaller included angle, the plug isshifted to present an opening in which the radius of this internalsurface is greater so that a smaller segment of its periphery is cutaway. This results in an air fan of smaller included angle and at thesame time results in a shifting of the point source away from the nozzleopening. The net effect, as far as matching the width of the paint fans,is thus similar to shifting the nozzle away from the paint film. As aresult, although its angle is smaller, the height of the air fan alongthe line of impingement of the paint film remains unchanged.Consequently, neither a large portion of the air stream is wasted bypassing outside the confines of the paint film, nor are portions of thefilm improperly atomized due to the fact that they are not impinged uponby the air stream. At the same time the distance the air has to travelto reach the zone of impingement is minimized.

These and other objects and advantages of our invention will be morereadily apparent from a consideration of the following detaileddescription of the drawings illustrating the principles involved in thepresent method of paint spraying and a preferred form of apparatus forcarrying out the method.

DESCRIPTION OF Til-IE DRAWINGS FIG. I is a diagrammatic view of a paintspray system embodying the present invention.

FIG. 2 is a semidiagrammatic elevational view of a paint spray issuingunder relatively low pressure from a fan nozzle.

FIG. 3 is a cross-sectional view of the paint spray taken along line 33of FIG. 2.

FIG. 3A is a diagrammatic view plotting the preferred distribution ofmass flow rate from an air nozzle used to atomize a paint spray film.

FIG. 41 is an enlarged top plan view of a portion of the paint sprayfilm shown in FIG. 2 illustrating the manner in which the paint spraytilm decreases in thickness as it travels away from the nonle.

FIG. 5 is a diagrammatic view showing the cooperative relationshipbetween a paint spray nozzle and an air spray nozzle in carrying out thepresent paint spray method.

FIG. 6 is a semidiagrammatic view taken along line 66 of FIG. 5.

FIG. 7 is a diagrammatic view showing the relationship of two paintnozzles and an air nozzle to carry out a preferred method of paintspraying in accordance with the present in vention.

FIG. 8 is a view taken along line fi-8 of FIG. 7.

1 FIG. 9 is a longitudinal sectional view of a preferred form of spraygun constructed in accordance with the present invention.

FIG. I0 is an enlarged cross-sectional view taken along line 10-10 ofFIG. 9 and with portions rotated for purposes of clarity.

FIG. 1 l is a view taken along line lll I of FIG. 10.

FIG. HA is a front elevational view of the air nozzle plug.

FIG. 12 is a cross-sectional view taken along line 12-I2 of FIG. 10.

FIG. 13 is a cross-sectional view taken along line 13l3 of FIG. 10.

FIG. 14 is a cross-sectional view taken along line l414 of FIG. 10.

FIG. 15 is a semidiagrammatic view showing the relationship of the airnozzle and paint film to a target.

FIG. 16 is a diagrammatic view similar to FIG. 15 showing the manner inwhich the air nozzle opening and spacing from the paint film are variedto decrease the width of paint spray.

FIG. 17 is a schematic circuit diagram of the safety circuit.

SYSTEM FIG. I shows a paint spray system I0 for spraying paints andother coatings in accordance with this invention. As is well recognizedin the art, coating systems of the general type shown are utilized toapply coatings to many different types of products, such as automobileparts, furniture, containers, and the like. The coating materialsutilized include not only paints, but also enamels, lacquers, stains,varnishes, emulsions, waxes, adhesives, and the like. In the followingdescription the word paint" will be used in a very generic sense toencompass all of these various types of coating materials.

In almost all instances, it is desirable that the paint or other finishbe applied in a smooth, even coating with the particles deposited on theworkpiece being of a small and relatively uniform size. Paint systemsfor carrying out the present process include a spray gun II which issupplied with paint by means of a pump 12 which feeds paint underpressure to the gun from a paint supply tank I3 through a paint tube I4.(Alternatively pump 312 can be eliminated and tank 13 pressurized). Thegun is also supplied with air under pressure from a conventionalcompressor or other air pressure source. The air is applied to the gunthrough an air tube I5.

As is well known in the art, when spraying some types of products, forexample, the interiors of deep tubular structures, it is desirable tooperate the system as a straight air spray system without applying anyelectrostatic field. However, in spraying other types of articles, it isdesirable to charge the paint particles so that the deposition of theparticles is aided by the presence of a high-electrostatic field. Insuch instances, the article being sprayed is normallymaintained atground potential so that the electrostatically charged paint particlesmove toward the article under the influence of the electrostatic fieldforces and a high percentage of them are effectively deposited on thearticle to be painted.

The present system can be operated either as a straight air spray gun oras an electrostatic air spray gun. In the event that the gun i1 is to beoperated as an electrostatic unit, it is connected to a power pack, orsource of DC voltage, 16. The power pack is connected to the gun throughan electrical cable 17.

Gun 11 may either be a hand-held gun or can be an automatic gun mountedon a suitable support normally positioned adjacent to a conveyor line bymeans of which the articles to be sprayed are moved past the gun. Ineither case, and particularly in the case of hand-held guns, it isdesirable that the gun itself be compact and maneuverable. With eitherform of gun the paint particles are atomized by a low-pressure stream ofair and are projected forwardly from the gun in a soft spray of fineparticles. A typical spray pattern of the present system travels onlyabout one-half of the travel of a conventional air spray gun operatedunder the same conditions.

PAINT SPRAYING METHOD In order to understand various ramifications ofthe present paint spraying process, it is desirable to consider thenature of a fan-shaped paint spray film as it is sprayed from a nozzleunder relatively low air pressure. The pattern of a typical paint filmI8 is illustrated in FIG. 2. The film is being emitted from a nozzle ofthe flat fan spray-type. Such nozzles are conventionally utilized inairless spray guns and one form is disclosed in Bede US. Pat. No.2,754,228. Another form of fan spray nozzle is disclosed in detailbelow. The paint film shown in FIG. 2 is being sprayed under arelatively low pressure, for example, a pressure of the order of 40-50p.s.i. as opposed to the normal pressure used in airless spray guns ofthe order of 3001 ,000 p.s.i.

As shown in FIGS. 2-4, the paint film is discharged from the orifice ina generally fan-shaped pattern with the upper and lower edges of thefilm diverging. The angle of divergence of these upper and lower edgesgradually decreases. The film is not uniform in cross section. Rather,for a short distance as it leaves the nozzle 20, the film includes acentral, relatively uniform web 21 and two edge enlargements, orstreamers 22 and 23. These beadlike streamers run along the upper andlower edges of the fan and are substantially thicker than the centralweb portion 21.

When low hydraulic pressures are used, such a film of viscous coatingmaterial remains relatively stable for a distance of perhaps one toseveral inches after it leaves the nozzle. However, at the end of thisstable region the film begins to form transverse ripples, or waves, 24.As the sheet becomes more, more and more unstable, the waves 24eventually disintegrate into ligaments and droplets 25. However, thestreamers persist and strongly resist disintegration.

In addition to these characteristics, the thickness of the filmdecreases progressively as the film leaves the nozzle. This decrease infilm thickness is generally inversely proportional to the distance fromthe nozzle so that the most rapid diminution of sheet thickness occursin the first fraction of an inch (e.g., one-quarter inch) of sheettravel from the nozzle and from that point on the rate of diminution issubstantially smaller.

The broadest aspect of the present paint spraying method is illustratedin FIGS. and 6. Essentially, we have determined that very fine paintatomization characterized by low mean particle size and freedom fromrandom large particles can be obtained by spraying a fan-shaped film ofpaint 27 under relatively low pressure, for example, a pressure of theorder of 3080 p.s.i., from a nozzle 28 and impinging this fan-shapedpaint spray with a fan-shaped stream of air 30 emitted under a very lowpressure of the order of 9-45 p.s.i. by an air nozzle 31. When the airstream impinges upon the paint fan, it exerts a strong shearing force onthe film and effectively atomizes all portions of the paint stream,including the streamers 32 and 33. The paint stream is deflected andcontinues outwardly as a soft spray pattern, or fog, 34 of suitablyatomized paint particlcs.

In accordance with the present method, the paint is struck by arelatively small volume of air at a relatively high velocity. We haveempirically determined that optimum atomization of particles for paintspraying is obtained when the angle x between the paint spray film andthe air spray is made between approximately 30 and 70. The line ofimpingement 35 between the air stream and paint stream is spaced farenough from the nozzle so that the paint stream has undergone a greaterportion of the attenuation of its thickness. At the same time, the lineof impingement 35 against a stream of the type shown in FIG. 2 shouldoccur within the region in which the paint film is substantially planar,i.e., before the transverse waves 24 would commence in the absence of anair stream.

Next, the length L, of the path of travel from the air male 31 to theline of impingement 35 should be only a fraction of the correspondinglength L of the paint film from the paint nozzle 28 to the line ofimpingement 35. We have empirically determined that the preferred rangeof ratio of L /L is from one-sixth to three-fourths. In one preferredembodiment, the

actual length of paint film L is approximately three-eighths tothree-fourths inch, while the actual length of air film L isapproximately one-eighth to three-eighths inch. The air stream isslightly wider (from edge-to-edge) along the line of impingement 35 ofthe paint film than is the paint film. This facilitates breaking up andeffective atomization of the streamers 32 and 33 which is essential toobtaining a good quality paint spray.

To further improve the atomization of the streamers, the air sprayitself is made nonuniform with the air stream having higher mass flowrates adjacent its edges, in the areas of the stream which impinge uponstreamers 22 and 23. This is shown diagrammatically in FIG. 3A in whichthe mass flow rate of the air is shown as being substantially greater inthe areas 36 and 37 in which the air stream impinges upon the streamers22 and 23 than in the central area 38 of the air stream which impingesupon the thin web portion 21 of the paint film.

In practice, this nonuniform air stream can be obtained by emitting airfrom a thin slot which is not of rectangular configuration, but ratheris wider adjacent to the ends. In many cases, however, the use of such aspecially configurated nozzle is not necessary since we have found thatthe use of a flat fan nozzle made with a rectangular gash gives asufficiently heavy edged airflow to atomize the heavy edges of the paintsheets.

In carrying out the present process, paint can be applied at a high ratewithout adversely afiecting the highly effective atomization achieved.Moreover, this fine and uniform atomization is achieved while utilizingonly a relatively low amount of air in relation to the quantity ofpaint. For example, on one typical operation an acrylic enamel having aviscosity of 22 seconds as measured by a Zahn No. 2 cup at roomtemperature was sprayed at a rate of 22 fluid ounces per minute. Thepaint pressure was approximately 40 p.s.i. and the air pressure was 20p.s.i. at the gun. The airflow was at the rate of 8 standard cubic feetper minute so that the ratio of air-topaint was 0.36 standard cubic feetof air per fluid ounce of paint sprayed per minute. Conventional airspray guns utilize an air-to-paint ratio approximately twice this largeand an air pressure of approximately 75 p.s.i. Consequently, the totalenergy of the air utilized in the present process is only a smallfraction of that required in a conventional air gun. This samerelatively low ratio of air volume-to-paint is utilized in the presentmethod through the entire range of viscosities of coating materialswhich are normally applied using the present method.

For example, a very viscous vinyl coating having a viscosity of 40seconds as measured on a Zahn No. 2 cup at room temperature wasdeposited at the rate of 17.2 fluid ounces per minute. The paintpressure was 45 p.s.i. and the air pressure was 26 p.s.i. at the gun.The volume of air employed was 9.5 standard cubic feet per minute. Thus,the ratio of air to ounces of paint was 0.55 standard cubic feet of airper minute pe fluid ounce of paint per minute.

As another example, a stain having a very low viscosity of 16.5 secondsas measured on a Zahn No. 2 cup at room temperature was applied at arate of 24 fluid ounces per minute. Again, the paint pressure was 45p.s.i. and theair pressure was 9 p.s.i. at the gun. The amount of airemployed to atomize this stain was 4.5 standard cubic feet per minute orapproximately 0.18 standard cubic feet of air per minute per fluid ouncepaint per minute. In each of the above two examples, the air utilized bya conventional spray gun was approximately twice the amount utilized inthe present spray gun.

An even more advantageous version of the present paint spraying processis disclosed in FIGS. 7 and 8. In this preferred method of sprayingpaint, two substantially identical fanshaped sprays of paint 40 and 41are emitted from fan spray nozzles 42 and 43. A single fan of air 44 isemitted from an air nozzle 45. The fan of air 44 is disposed so that itbisects the angle formed by the two planar fans of paint 40 and All. Thetwo fan streams of paint converge and are impinged by the air stream Min an area indicated generally at 47. Both paint streams are broken upinto a fine mist, or fog, of generally uniform paint particles asindicated in FIG. 8. In fact, the atomization is even more efficientthan in the embodiment disclosed in FIGS. and 6 since the air stream isin effect trapped between the two fluid streams and its energy is moreeffectively utilized in breaking up the paint into minute droplets.

The same parameters described above in connection with the methodillustrated in FIGS. 5 and 6 are also applicable to the method disclosedin FIGS. 7 and 8. More particularly, the angles Y and Y between thepaint and air streams are in the range of from approximately 30 toapproximately 70. The paint pressure is from 30 to 80 p.s.i., while theair pressure is from 9 to 45 p.s.i. at the gun. The ratio of the lengthof air travel L,, from the nozzle to the zone of impingement compared tothe length of paint travel from the nozzle to the zone of impingement isapproximately in the range of one-sixth to three-fourths. Also, theactual length of paint travel L is of the range of approximatelythree-eighths to three-fourths inch, while the actual length of air filmtravel L is approximately one-eighth to three-eighths inch. Furthermore,the ratio of volume of air to quantity of paint applied is the same asthat described above. This ratio is in the range of from 0.2 to 0.6standard cubic feet of air per fluid ounce of paint.

The process described above can also be employed to effect asimultaneous atomizing, mixing and spraying of two different liquids forcoating. More particularly, it has been determined that one type ofliquid can be sprayed from one nozzle and a different type of liquid canbe sprayed from the second nozzle with the result that the coatingdeposited on the work is of a uniform blend of the two coating liquids.

The process of atomization described thus far can be employed by itselfto cause effective application of paint or a similar coating to aproduct. However, the atomization process can also be utilized inconnection with a paint charging step to electrostaticaily spray paint.In accordance with the present invention, this charging operation iseffected by utilizing the atomizing air as a charge carrier fortransferring charges to the paint particles.

As is shown in FIG. 7, an electrode 50 is disposed in the air streampreferably within the air nozzle 45. This electrode is connected to asource of high DC potential, for example, a potential of the order of 50kv. It will, of course, be understood that the potential applied to theelectrode 50 can be greater or less than this figure, depending uponsuch variables as the characteristics, e.g., resistivity of the paintbeing applied, nature of the article being coated, distance of the gunfrom the article, etc.

The potential applied to electrode 50 causes a corona discharge to beformed surrounding the tip of the electrode. The. air stream passesthrough this corona on its path through the nozzle andas a result largenumbers of ions are formed in the stream. These ions, i.e., ionized gasmolecules have relatively high mobility and move primarily under theinfluence of the electrostatic field toward the coating material at arelatively low potential. In practice, the ions thus move toward thepaint which is at a low potential and ultimately attach themselves tothe atomized paint particles.

The two paint stream method illustrated in FIGS. Iv and 8 results in animproved charging efficiency over the one paint stream method shown inFIG. 5. One possible explanation for this is that the paint streamsenvelope the air stream, i.e., are disposed on both sides of it.Consequently, the ions must pass through a paint stream or a fog ofpaint particles in order to reach a ground surface. In this travel thereis a high likelihood that they will attach themselves to one of thepaint particles. In contrast, in the one paint stream method of FIG. 5,the ions can escape to a grounded surface without passing either througha paint stream or a fog of particles, e.g., as shown in FIG. 5 thecharged ions could be attracted to a grounded surface and move off tothe right without ever contacting the paint.

In any event, empirical tests have shown that the two paint fanelectrostatic paint spraying method of FIGS. 7 and 8 as compared to theone paint fan method of FIG. 5 (even when the air stream of the onepaint fan method is ionized).

Another important facet of the present spraying method involves thecontrol of the width of the spray pattern. More particularly, it will beappreciated that articles being sprayed vary in physical dimensions sothat in order to minimize overspray, it is ofien desirable to alter thesize of the paint spray pattern produced by the gun. The novel manner inwhich this is accomplished in the present invention is diagrammaticallyshown in FIGS. 15 and 16.

FIG. 15 shows a paint spray directed against a target 55. The actualpattern 56 of spray has a width W which in actual practice would beslightly longer than the target 55. We have empirically determined thatthe width of the spray pattern can be varied by changing the includedangle S of the air stream without changing the width of the paintstream.

As shown in FIG. 15, a fan-shaped sheet of air 57 is emitted from anozzle 58. The air impinges upon a fan-shaped film of paint 60. It willbe appreciated that this film of paint may be a single film which isprojected at an angle to the air fan as shown in FIGS. 5 and 6, or mayin fact be constituted by two paint films as shown in FIGS. 7 and 8.

In either event, the air is projected from the nozzle in a fanshapedstream which is shown as having upper and lower edges 61 and 62including an angle S such that the air stream just intercepts the entiresection of the film against which the air stream impinges. Again, itwill be appreciated that in actual practice the angle S would beslightly greater than that shown so that the air stream passes slightlyabove and below the film. (This exact relationship is not shown,however, to simplify the present explanation.)

In practice, the spacing between the paint film 60 andthe target 55remains substantially constant. Accordingly, when it is desired to spraya smaller target 55, as shown in FIG. 16, it is desirable to produce apaint pattern 56' having a smaller width W' without requiring shiftingof the gun. In accordance with the present invention, this change in thespray pattern is accomplished by projecting the air fan at a smallerangle S. It will be appreciated that if the angle of the air fan in FIG.15 were reduced with no further change, the air stream would intersectonly a portion of the paint film 60. This would result in substantiallyunatomized, or coarsely atomized, paint and, hence, a defective finish.

In accordance with the present method, this difficulty is avoided andthe paint pattern width is varied without affecting the uniformatomization of all of the paint. Specifically, the size of the paintpattern is altered by making two changes in the air fan. In the firstplace, the included angle of the air stream is reduced from the angleS-to the angle S. In the second place, the internal configuration of thenozzle opening is changed so that the apparent focal point, or pressuresource point, P of the air stream is shifted away from the nozzleopening and away from the paint film to a more remote point as indicatedby F.

More particularly, as shown in FIG. 15, it is apparent that the nozzleopening is formed by the intersection of two segments of circular arcs,an external are 63 and an internal arc 64. In this example, the arcs aredisposed relative to one another so that the nozzle opening encompassesapproximately l80 of the inner are 64. This nozzle functions as thoughthe air emanates from a point source P disposed relatively close to thenozzle opening.

The changed nozzle opening, as shown in FIG. 16, is also formed by theintersection of outer arc 63' and inner arc 64'. In this case, theradius of the outer arc 63' is the same as that of are 63. However, theradius of the inner arc 64 is appreciably greater than the are 64. As aresult, the nozzle opening intersects a much smaller portion of theinner arc 64'. This nozzle functions as though the air emanates from apoint pressure source I spaced an appreciable distance from the nozzleopening and from film sheet 60'.

As a result of these concomitant changes, i.e., lessening the includedangle of the air fan while at the same time shifting the apparent focalpoint or apparent point source of the air stream away from the nozzleand paint film, the height of the air fan at its point of intersectionwith the film remains constant. Consequently, by changing the air fan inthis manner without changing any other aspect of the operation of thegun, for example, the paint pressure or paint sheet size, the paintpattern can be compacted or enlarged. A preferred form of nozzleconstruction for effecting these changes in a simple manner isillustrated in FIGS. -14, and the details of construction of this nozzleare explained below.

SPRAY GUN The details of construction of a preferred form of spray gunfor carrying out the present method of paint spraying are shown in F168.9-14. As there shown, the spray gun 11 comprises a hollow handle portion66, a barrel or extension portion 67 formed of a suitable insulatingmaterial and mounted forwardly of the handle and a nozzle assembly 68mounted on the forward end of barrel 67.

The nozzle assembly 68 includes two paint nozzles 70 and 71 effective toproject flat fan-shaped sprays of paint toward the center line of thegun as indicated by dotted lines 72 and 73. The gun further includes anair nozzle 74 effective to project a fan-shaped stream of air forwardlyalong the axis of the gun toward the area of intersection of the paintstreams as indicated by line 75. Additionally, the gun includes anelectrostatic charging circuit including a needle electrode 76 forcharging the atomizing air and ultimately causing a charge to be appliedto the atomized paint particles.

Operation of the gun is controlled by a single trigger 77 which iseffective to control flow of paint through a paint valve 78, flow of airthrough air valve 80, and application of electric potential to electrode76 through actuation of switches 81 and 82. The trigger functions tofirst open air valve 80, then open paint valve 78 and finally closeswitch 81. When the trigger is released this sequence is reversed.

More particularly, handle 66 is formed of a suitable conduc- Livematerial, such as aluminum or the like. The handle is hollow and isconfigurated to form a hand grip portion 83, a forwardly extending stockportion 84 and a suspension hook portion 85. A transverse flange isformed at the forward end of the stock for abutment with a mating flangeon the barrel. The barrel and stock flanges are bolted together in aconventional manner. Hand grip portion 83 is provided adjacent its lowerend with an opening 86 for receiving the combined air and electricalinput fitting 87.

Air is introduced to the gun through an air tube which surroundselectrical cable 17, there being an annular space between the tube walland cable for passage of air from the compressor to the inlet fitting ofthe gun. The hollow interior of the gun is sealed at the cable entrancein any suitable manner, such as by means of an Oring 90 compressedbetween a shoulder in the gun housing and a threaded cap member 91. Airpasses through an inlet passageway 92 formed in the handle and enters avalve chamber 93 of air valve 80.

This valve includes a reciprocating stem member 94 having an extension95 disposed for abutment with trigger member 77. An intermediate portionof the stern carries an O-ring 96 or similar packing member disposedbetween spaced shoulder 97. The inner end of the stem carries a taperedplug 98 adapted to engage a tapered seat to seal off the air flow. Whenthe trigger is depressed, plug 98 is shifted against the force of spring99 from engagement with seat 100 and air is free to flow into internalchamber 101 in the gun handle.

This chamber communicates with a longitudinal bore 102 in the stock anda continuation of this bore 103 formed in the barrel member. Thischamber surrounds a sleeve 104 which is formed of insulating materialand is supported in mounting spiders 105 and 106. The forward end ofbore 103 communicates with a longitudinal bore 107 formed in nozzlesupport member 108 which carries the paint and air nozzles. Nozzlesupport member 108 is preferably formed of an insulating material suchas Delrin. Support member 108 is mounted on the front end of barrelmember 67 by means of a threaded cap 109 which is coupled to themounting member by a retaining ring 119 titted in opposed grooves in thecap and mounting member. Cap 109 engages threaded extension 129 on thebarrel.

Support member 108 is provided with a circular plugreceiving opening110. This opening 110 houses a generally cylindrical air nozzle plug 111which is preferably made of a plastic material, e.g., Delrin. Nozzleplug 111 includes an annular peripheral wall 112 having an arcuate inletopening 113 which communicates with a vertical central chamber 1 14. Thelower end of the plug is closed by a solid circular wall 115, while thetop of the plug is closed by a cap 116 which is press fit or otherwisesecured to the top of the plug. The cap seals off the upper end ofchamber 114 and includes an outwardly extending peripheral flange 117having a transverse slot 118 for rotating the cap and plug member.

The peripheral wall of the plug member opposite entrance slot 113 isprovided with three parallel spaced, rectangular openings 120, 121 and122. The plug can be rotated to selectively align any one of theseopenings with front opening 123 in member 108. The selection of theparticular nozzle opening 120, 121 and 122 disposed in registry withfront opening 123 of the stationary support member 108 provides themeans for varying the air stream to vary the paint spray pattern in themanner shown diagrammatically in FIGS. 15 and 16.

More particularly, opening is effective to provide the widest angle ofair spray fan and, hence, the widest paint spray pattern. Similarly,opening 121 is effective to provide the narrowest air spray fan and,hence, the narrowest paint spray pattern. Noule opening 122 is effectiveto provide an air spray fan and paint spray pattern of intermediatesize.

It will be noted that nozzle opening 120 is formed by a semicylindricalinternal cut 124 formed in peripheral wall 112, the axis of cut 124being horizontal. This cut is intersected by a segmental disclike cut125 oriented in a vertical plane and extending inwardly from the outerperiphery of wall 112 internally along a radius of the plug member.

In one preferred embodiment of the plug member, the disclike cut 125 hasa 0.375-inch radius with a center located 0.178 inch outwardly from theperiphery of the plug wall. Intemal cut 124 has a 7/64-inch radiuslocated inwardly 0.169 inch from the periphery of the plug. This nozzleopening has been found effective to emit a fan-shaped stream of airhaving an included angle of approximately 94. Hydraulically, this nozzleopening functions generally as though there were a point source of gaspressure located at point P1 approximately one-fourth inch inwardly fromthe periphery of the plug member.

Slot 121 is formed by an internal semicylindrical cut 126 formed inperipheral wall 112 along a horizontal axis. Cut 126 is intersected by ahorizontal disclike cut 127. Disclike cut 127 extends radially inwardlyfrom the outer periphery of wall 112. In one preferred embodiment ofplug, the disclike cut 127 has a 0.375-inch radius with the centerlocated 0.2l9 inch outwardly from the periphery of the plug wall.

internal cut 126 has a radius of 0.203 inch with the center of the cutbeing located 0.263 inch from the periphery of the plug. This nozzleopening has been found to emit a fan-shaped stream of air having anincluded angle of approximately 36. This nozzle functions generally asthough the air emitted from it emanated from a point source P3 disposedsubstantially further from the peripheral wall than point P1 (e.g.,approximately four times as far).

in order to maintain the volume of air emitted from nozzle openings 120and 121 substantially equal, the width of nozzle opening 121 is madeslightly narrower than the width of nozzle opening 120 so that thecross-sectional area of the opening is constant. Thus, in one preferredembodiment, the width of nozzle opening 120 is made equal to 0.074 inch,while the width of nozzle opening 121 is made equal to 0.065 inch.

The third, or intermediate size, nozzle opening 122 is similarly formed,i.e., this nozzle opening is formed from asegment of a cylindricallyinternal cut 128 intersected by a vertical disclike cut 130, disclikecut 130 extending inwardly along a radius from the outer peripheral wall112. In one preferred embodiment, the cut 130 has a 0.375-inch radiuswith a center located 0.168 inch from the periphery of the plug. Innercut 123 has a radius of 0.156 inch located 0.216 inch from the peripheryof the disc. This nozzle opening emits a fan-shaped stream of air havingan included angle of approximately 64. Air is emitted from this nozzlein generally the same manner as though the air flowed from a pointsource P2 located of the order of twice the distance from the peripheryof the plug as point P1. In this preferred embodiment the pressure ofthe air applied to the gun is relatively low, for example, of the orderof 9-45 psi. at the gun. The exit velocity of the air is approximatelysonic.'The distance, in the preferred gun, of air travel L to the pointof impingement of the air and paint streams is approximatelythree-sixteenths inch.

Paint is supplied to the gun from pump 12 through tube 14. This tube iscoupled to the gun through fitting 131 which communicates with alongitudinal paint passage 132 formed in extension member 67. Passage132 communicates with the seat area 133 of paint valve 78. This seatarea is engaged by a spherical head 134 of valve stem member 135. Thisvalve stem member reciprocates longitudinally to shift the head 134toward and away from the seat member and is spring-urged toward the seatby means of a compression spring 136.

The valve is adapted to be opened by pressure on trigger 77. When thetrigger is pivoted toward the handle member about pivot pin 137 whichsupports the upper end of the trigger, the trigger engages a shoulder138 secured to stem 135, forcing the stem rearwardly to open the paintvalve. With paint valve 78 open, paint is free to flow throughlongitudinal conduit 140 into annular passageway 141 and from thispassageway into axial angulated bores 1 12 and 143 formed in nozzlesupport member 108. These bores communicate with paint nozzle assemblies70 and 71.

More particularly, bores 142 and 143 are aligned with 1111818 1 M and145 in cylindrical members 1416 and 1417. Each of these cylinders isformed with an internal passageway 148 and a seat 150 for a check valveformed by ball member 151. Each of the balls 151 is spring-urged againstthe seat by means of a spring 152 bearing against a thrust washer 153mounted within a chamber 156 in cylindrical members 116, 1417. Each ofthe chambers opens into an axial passageway 155 communicating with anaxial bore 156 in tubular nozzle member 157. Each of the nozzle membersis preferably formed of a suitable plastic material, such as Delrin.

The fluid discharge passageway in each of the nozzles is formed by aV-shaped cut 158 extending inwardly from a periphery of the wall toapproximately the centerline of the tube. Each of the V-shaped cuts 153includes a rearward wall 160 and a forward wall 161. As shown in FIG.10, each nozzle 157 is angulated, or tilted, slightly outwardly relativeto the axis of passageway 155. This tilting enables the nozzle to directthe paint stream at the desired angle relative to the air fan.

More particularly, each nozzle 157 is effective to emit a thinsubstantially planar fan-shaped stream of paint at an angle ofapproximately 60 to the axis of bore which is parallel to the axis ofthe gun. Consequently, this fan-shaped paint stream is impinged by thefan-shaped air stream at approximately the same angle, i.e., atapproximately 60. In this preferred embodiment, the length of painttravel L from the nozzle to the I zone of impingement of the paint andair streams is approximately five-eighths inch.

In accordance with the present invention, the ends of axial bores 156 ineach of the nozzle tubes 157 is closed by a springlosded plug 159. Theseplugs are secured to angulated spring members 162 in any suitablemanner; for example, as shown in FIG. 10, plug 159 passes through asmall bore in a portion of arm 162 and has an overturned head .163effective to clamp the plug to the arm. Both plugs 159 and springmembers 162 are preferably formed of a suitable plastic material. Arms162 include a transverse section 164 and a foot portion 165 which ismounted upon the support member 108 in any suitable manner, such as bymeans of bolts 166.

Each of the spring members 162 also includes an overturned flangeportion 167 by means of which the spring arm can be pulled outwardly todisengage the plug from the seat formed in the end of the associatedtube member 157. In normal operation, the spring force is sufficient tomaintain the plug in sealing engagement with the end of the tube 157 However, in the event that the nozzle should become clogged, the

passage can readily be cleared by grasping flange portion 163 to pullthe spring arm and plug outwardly, opening the end of the tube. Trigger77 is then depressed to cause paint to be discharged from the end of thetube removing whatever impediment had accumulated in the tube section.

The electrical input to the gun is preferably through a highresistancecable 17. The details of construction of one preferred form of cable aredisclosed in Rosen US. Pat. No. 3,348,186. The remote end of this cableis connected to a power pack 16 effective to develop a high DCpotential, for example, 50,000 volts. Cable 17 enters the gun throughthe interior of fitting 87 and passes upwardly through a hollow section170 of the handle. The cable is flexible and is bent rearwardly aroundshoulder 171 at the junction of the hand grip portion 83 and the stockportion 84 toward the rear wall 172 of the stock. The cable is then bentforwardly and is inserted into insulating sleeve 104, the cable beingloosely received in the sleeve so that it is free to move relativethereto. The forward end of the cable is provided with a contact button173 disposed for mating contact with a similar contact button 174provided on the rear end of cylindrical resistor 175.

This resistor is preferably sealed in the end of sleeve 104 and has anelectrical resistance of the order of 75 megohms. The forward end ofresistor 175 is in electrical contact with a bolt 176 passing inwardlythrough a cap member 177 enclosing the forward end of sleeve 104.

The head 180 of bolt 176 is engaged by a contact spring 178 which iscompressed between the head of the bolt and a head 181 formed onelectrode 76. Electrode 76 is in the form of an elongated pin supportedin axial aiignment with the air nozzle by means of a cruciforminsulating member 182. This cruciform insulating member includes acentral bore 183 into which the pin is press fit. The cruciform memberis rigidly mounted in the central opening 107 in support member 105.

Electrode '76 extends forwardly through entrance opening 113 in the airnonle member and extends forwardly to a position just inside theperipheral wall 112 carrying the three nozzle openings 120, 121, and122. In normal operation, this electrode is charged to a suitablepotential of the order, for example, of 40,000 volts. A corona isestablished surrounding the tip of the electrode. The stream of airflowing through the nozzle surrounds the electrode and passes throughthe corona prior to the time it is discharged from the nozzle opening.As the air passes through the corona gasmolecules are ionized and dustparticles are charged. These ions and charged particles move forwardlyunder the influence of the electrostatic field forces toward a lowerpotential region of the field constituted by the paint films andatomized particles. There the charges attach themselves to the paintparticles to form the charged paint spray.

One of the advantages of this paint charging device is that the paintsupply in conduit 14 does not become charged so that the entire paintsupply system, including tube 14, pump 12 and the paint supply tank 13,remain at or very near ground potential. In fact, in the embodimentshown the paint supply system is grounded through needle 135 and handlemember 66 which is conventionally grounded in use. This greatlysimplifies the problem of insulating the paint supply system andeliminates the dangers common in many prior art systems due to chargeaccumulation in the paint supply system.

In addition to the elements described above, the electrical chargingcircuit includes a safety circuit for preventing application of power tothe cable in the event that resistor 175 is removed from the gun and thegun reassembled without replacing the resistor.

More particularly, as is best shown in FIG. 9, the cable 17 forms arearwardly extending resilient bend 190 adjacent to rear wall 172 of thestock. An actuating arm 191 is pivoted about pin 192 in the stockportion of the gun and is interposed between cable bend 190 and rearwall 172. This arm includes a forwardly extending link portion 189 whichis attached to one end of tension spring 193, the other end of thespring being secured to a stationary bracket 194.

Bracket 194 carries snap-action microswitches 81 and 82. Microswitch 82is normally closed, while microswitch 81 is normally open. A spring arm195 is disposed for engagement with actuating plunger 196 of microswitch82. This spring arm is adapted to be shifted with link 189, for example,by engagement with a shoulder 197 carried by the link.

So long as the cable 17 is positioned rearwardly as shown in FIG. 9,plunger 1% is not depressed and switch 82 remains closed. In normaloperation of the gun, the cable is held in this position against forwardmovement by the presence of resistor 175. However, if resistor 175 andsleeve 104 are not present in the gun, spring 193 acting upon arm 191forces the cable forwardly within conduit 103. This forward movement ofarm 191 causes arm 195 to open switch 82.

As is shown in FIG. 17, switch 82 is connected in series with switch 81and a coil 198 of relay 200. Switch 81 is normally open switch adaptedto be closed upon actuation of trigger 77. More particularly,microswitch 81 includes an actuating plunger 201 adapted to be engagedby a spring arm 202. This arm in turn is positioned for contact with ahead 203 formed on the continuation of stem member 135.

When trigger 77 is depressed, the stem and its extension are shiftedrearwardly so that head 203 bends spring arm 202, closing microswitch81. When both microswitch 81 and 82 are closed, relay 200 is energizedto complete a circuit to power pack 16. This power pack is theneffective to apply a potential to cable 17 and, hence, to electrode 76.

The safety circuit just described is also effective to preventapplication of power to the cable when the nozzle assembly is removedfrom the gun. More particularly, as is shown in FIG. 9, tube 104 issupported in the surrounding bores in the stock member and barrel bymeans of low friction spiders 105 and 106. The tube is normally retainedin its retracted position shown in FIG. 9 by the abutment of spider 182with the end of cap member 177. However, when the nozzle mounting member108 is removed by unthreading cap 109 from the threaded extension 129 onthe barrel member, the restraint on forward movement of the sleeve isremoved. The sleeve is then shifted forwardly under the influence ofspring 193 acting on the cable member.

As the cable member is shifted to the left, arm 191 is shifted forwardlycausing arm 195 to open switch 82 preventing energization of the powerpack and thereby preventing application of potential to cable 17. At thesame time, forward movement of sleeve 104 is effective to provide asecond function in that it automatically seals the air chamber and endof the resistor or electrical conduit from contamination. Moreparticularly, as best shown in FIGS. 9 and 10, the forward end of cap177 is tapered to form a seat 204. This seat is effective to engage amating tapered seat 205 formed in the barrel 67 when sleeve 104 isshifted forwardly, i.e., to the left in FIG. 10.

When the nozzle is reassembled, spider 182 engages cap 177 forcing thecap away from the seat 205 to reopen the air conduit. At the same time,the cable 17 is shifted rearwardly so that bend causes am 191 to pivot,thereby reclosing switch 82 to permit reenergization of the power packand cable 17 when trigger 77 is depressed.

The construction of the gun as described above also incorporates twoadditional safety features. In the first place, switches 81 and 82 andthe other electrical contacts such as those between the cable andresistor occur in an enclosed or pressurized explosiveproof chamber.Moreover, this chamber is continuously purged to remove any accumulatedgases by the flow of the air utilized to atomize the paint.

From the foregoing disclosure of the general principles of the presentinvention and the above description of a preferred embodiment, thoseskilled in the art will readily comprehend various modifications towhich this invention is susceptible.

For example, while the air fan and paint fan streams have been describedas being substantially planar, it is contemplated that the paint fansmay be slightly arcuate, i.e., in the form of thin bowed sheets.Moreover, if a gun is to be utilized for atomizing, mixing and sprayingtwo different liquids by spraying one liquid from one nozzle and anotherliquid from the other nozzle, it is apparent that a separate supply linemust be provided for each nozzle. This can readily be accomplished byessentially duplicating the liquid supply system shovm while eliminatingthe conduit which now interconnects the two nozzles. Also while air hasbeen described as the atomizing gas, it will be appreciated that in someparticular installations other gases could be employed. Accordingly, theterm air" as used in the following claims should be interpreted asincluding other gases as well.

Accordingly, we desire to be limited only by the scope of the followingclaims in which it is to be understood that the term paint is used inthe generic sense to cover finish coatings of the type described aboveand in which the term gun is used generically to cover either a handspray gun or a mechanically supported control spray device.

Having described our invention, we claim:

1. A paint spray gun, said gun comprising,

a handle portion,

a barrel portion extending forwardly of said handle portion, said handleportion being hollow and having an air passageway therethrough,

an air conduit within said barrel portion in communication with the airpassageway in said handle,

a paint conduit in said barrel portion,

a paint nozzle in communication with said paint conduit,

an air nozzle in communication with said air conduit,

an electrode for causing said paint particles to becomeelectrostatically charged,

means for applying a potential to said electrode,

said means comprising a cable, said cable entering said gun through saidhandle and including a bent portion being disposed within said stock anda forwardly extending portion disposed within said barrel, a resistor inabutment with the end of said cable and extending forwardly thereof,said resistor providing an electrical interconnection between said cableand said electrode, spring means urging said cable toward said resistor,a switch effective to control the voltage applied to said cable, saidswitch being disposed for actuation to deenergize said cable when saidcable is shifted forwardly under the influence of said spring uponremoval of said resistor.

2. The paint spray gun of claim 1 in which a sleeve is slidably disposedin said air conduit and said resistor and said cable are disposed withinsaid sleeve, a mounting member is disposed on the end of said barrel forsupporting said air nozzle, said paint nozzle and said electrode, saidmounting member carrying means for restraining forward movement of saidsleeve, whereby when said mounting member is removed said cable movesforwardly and said switch is actuated to deenergize said cable.

3. The paint spray gun of claim 2 in which said barrel is configuratedto form a seat and the forward end of said sleeve is configurated toform a plug, whereby when said mounting member is removed said sleevemoves forwardly and said plug engages said seat to seal said airconduit.

4. The spray gun of claim 1 in which said switch is disposed within theair passageway in the handle, said air conduit constitutes an explosiveproof chamber.

5. The spray gun of claim 1 further comprising a second switch forcontrolling energization of said cable,

a trigger for actuating said second switch, said first named switch andsecond switch being disposed within said air passageway, said airpassageway constituting an explosive proof chamber.

6. A paint spray gun for use in a system including a source ofcompressed air and a source of paint,

said gun having a barrel portion,

an air conduit through said barrel portion, means for connecting saidair conduit to said source of compressed air,

a paint conduit through said barrel portion, means for connecting saidpaint conduit to said source of paint,

a flat fan air nozzle in fluid connection with said air conduit,

a flat fan-type paint nozzle,

means supporting said air nozzle and said paint nozzle on said barrel,

said air nozzle and said paint nozzle being in fluid communication withsaid air conduit and said paint conduit respectively,

said air nozzle being effective to project a flat fan of air forwardlyof said gun,

said paint nozzle being effective to project flat fan of paint forwardlyof said gun which intersects said flat fan of air in a zone spacedforwardly of said gun,

means for ionizing said air in an area rearwardly of said zone in whichsaid air fan impinges upon said paint fan,

whereby the air fan is effective to atomize said paint and to chargesaid paint to form a charged paint spray.

7. The paint spray gun of claim 6 in which said means for ionizing saidair comprises an electrode disposed within said air nozzle.

8. The paint spray gun of claim 6 in which the paint nozzle is disposedso that the flat fan of paint intersects the stream of air at an angleof from 30 to 70v 9. The paint spray gun of claim 6 in which the air andpaint nozzles are disposed so that the length of air travel of the airfan from the nozzle to the zone of air-paint impingement is fromone-sixth to three-fourths the length of travel of the paint fan fromthe paint nozzle to the zone of air-paint impingement.

110. The paint spray gun of claim 6 in which the air and paint nozzlesare disposed so that the air fan travels a distance of approximatelyone-eighth to three-eighths inch from the air nozzle to the point ofair-paint impingement.

ill. The paint spray gun of claim 6 in which the air and paint nozzlesare disposed so that the paint fan travels from approximatelythree-eighths to three-fourths inch from the paint noz zle to the zoneof air-paint impingement.

112. The paint spray gun of claim 6 in which said air nozzle comprises ahousing having a cylindrical chamber formed therein and an elongatedexit slot and an entrance opening communicating with said chamber, ashiftable plug disposed within said chamber, said shiftable plugincluding a plurality of nozzle slots formed therein, each said nozzleslot being effective to limit a fan of air having a different includedangle, a

central bore communicating with said nozzle slots and an inlet openingcommunicating with said bore, said plug being shiftableto selectivelyalign any of said exit slots with the slot in said housing, and anelectrode extends inwardly through the entrance opemng in said housingand the inlet opening of said plug, said electrode being disposed inalignment with the exit slot of said chamber and the nozzle slot of saidplug in rem'stry therewith.

13. A paint spray gun for use in a system including a source ofcompressed air and a source of paint,

said gun having a barrel portion,

an air conduit through said barrel portion, means for connecting saidair conduit to said source of compressed air,

a paint conduit through said barrel portion, means for connecting saidpaint conduit to said source of paint,

a flat fan air nozzle in fluid connection with said air conduit,

a pair of flat fan-type paint noules,

means supporting said air nozzle and said paint nozzles on said barrel,

said air nozzle and said paint nozzles being in fluid communication withsaid air conduit and said paint conduit respectively,

said air nozzle being effective to project a flat fan of air forwardlyof said gun,

said paint nozzles being efiective to project flat fans of paintforwardly of said gun which intersect said flat fan of air in a zonespaced forwardly of said gun, means for ionizing said air in an arearearwardly of said zone in which said air fan impinges upon said paintfans, whereby the air fan is effective to atomize said paint and tocharge said paint to form a charged paint spray.

14. The paint spray gun of claim 13 in which the paint nozzles aredisposed so that the flat fans of paint intersect the stream of air atan angle of from 30 to 70.

15. The paint spray gun of claim 14 in which the air and paint nozzlesare disposed so that the air fan is disposed substantially on thebisector of the angle between said paint fans.

16. The paint spray gun of claim 13 in which the air and paint nozzlesare disposed so that the length of air travel of the air fan from thenozzle to the zone of air-paint impingement is from one-sixth tothree-fourths the length of travel of the paint fans from the paintnozzles to the zone of air-paint impingement.

17. The paint spray gun of claim 13 in which the air and paint nozzlesare disposed so that the air fan travels a distance of approximatelyone-eighth to three-eighths inch from the air nozzle to the point ofair-paint impingement.

18. The paint spray gun of claim 13 in which the air and paint nozzlesare disposed so that each paint fan travels from approximatelythree-eighths to three-fourths inch from the paint nozzle to the zone ofair-paint impingement.

19. The paint spray gun of claim 13 in which said means for ionizingsaid air comprises an electrode disposed within said air nozzle.

20. The paint spray gun of claim 13 in which said air nozzle comprises ahousing having a cylindrical chamber formed therein and an elongatedexit slot and an entrance opening communicating with said chamber, ashiftable plug disposed within said chamber, said shiftable plugincluding a plurality of nozzle slots formed therein, each said nozzleslot being effective to limit a fan of air having a different includedangle, a central bore communicating with said nozzle slots and an inletopening communicating with said bore, said plug being shiftable toselectively align any of said exit slots with the slot in said housing,and an electrode extends inwardly through the entrance opening in saidhousing and the inlet opening of said plug, said electrode beingdisposed in alignment with the exit slot of said chamber and the nozzleslot of said plug in registry therewith.

1. A paint spray gun, said gun comprising, a handle portion, a barrelportion extending forwardly of said handle portion, said handle portionbeing hollow and having an air passageway therethrough, an air conduitwithin said barrel portion in communication with the air passageway insaid handle, a paint conduit in said barrel portion, a paint nozzle incommunication with said paint conduit, an air nozzle in communicationwith said air conduit, an electrode for causing said paint particles tobecome electrostatically charged, means for applying a potential to saidelectrode, said means comprising a cable, said cable entering said gunthrough said handle and including a bent portion being disposed withinsaid stock and a forwardly extending portion disposed within saidbarrel, a resistor in abutment with the end of said cable and extendingforwardly thereof, said resistor providing an electrical interconnectionbetween said cable and said electrode, spring means urging said cabletoward said resistor, a switch effective to control the voltage appliedto said cable, said switch being disposed for actuation to deenergizesaid cable when said cable is shifted forwardly under the influence ofsaid spring upon removal of said resistor.
 2. The paint spray gun ofclaim 1 in which a sleeve is slidably disposed in said air conduit andsaid resistor and said cable are disposed within said sleeve, a mountingmember is disposed on the end of said barrel for supporting said airnozzle, said paint nozzle and said electrode, said mounting membercarrying means for restraining forward movement of said sleeve, wherebywhen said mounting member is removed said cable moves forwardly and saidswitch is actuated to deenergize said cable.
 3. The paint spray gun ofclaim 2 in which said barrel is configurated to form a seat and theforward end of said sleeve is configurated to form a plug, whereby whensaid mounting member is removed said sleeve moves forwardly and saidplug engages said seat to seal said air conduit.
 4. The spray gun ofclaim 1 in which said switch is disposed within the air passageway inthe handle, said air conduit constitutes an explosive proof chamber. 5.The spray gun of claim 1 further comprising a second switch forcontrolling energization of said cable, a trigger for actuating saidsecond switch, said first named switch and second switch being disposedwithin said air passageway, said air passageway constituting anexplosive proof chamber.
 6. A paint spray gun for use in a systemincluding a source of compressed air and a source of paint, said gunhaving a barrel portion, an air conduit through said barrel portiOn,means for connecting said air conduit to said source of compressed air,a paint conduit through said barrel portion, means for connecting saidpaint conduit to said source of paint, a flat fan air nozzle in fluidconnection with said air conduit, a flat fan-type paint nozzle, meanssupporting said air nozzle and said paint nozzle on said barrel, saidair nozzle and said paint nozzle being in fluid communication with saidair conduit and said paint conduit respectively, said air nozzle beingeffective to project a flat fan of air forwardly of said gun, said paintnozzle being effective to project flat fan of paint forwardly of saidgun which intersects said flat fan of air in a zone spaced forwardly ofsaid gun, means for ionizing said air in an area rearwardly of said zonein which said air fan impinges upon said paint fan, whereby the air fanis effective to atomize said paint and to charge said paint to form acharged paint spray.
 7. The paint spray gun of claim 6 in which saidmeans for ionizing said air comprises an electrode disposed within saidair nozzle.
 8. The paint spray gun of claim 6 in which the paint nozzleis disposed so that the flat fan of paint intersects the stream of airat an angle of from 30* to 70*.
 9. The paint spray gun of claim 6 inwhich the air and paint nozzles are disposed so that the length of airtravel of the air fan from the nozzle to the zone of air-paintimpingement is from one-sixth to three-fourths the length of travel ofthe paint fan from the paint nozzle to the zone of air-paintimpingement.
 10. The paint spray gun of claim 6 in which the air andpaint nozzles are disposed so that the air fan travels a distance ofapproximately one-eighth to three-eighths inch from the air nozzle tothe point of air-paint impingement.
 11. The paint spray gun of claim 6in which the air and paint nozzles are disposed so that the paint fantravels from approximately three-eighths to three-fourths inch from thepaint nozzle to the zone of air-paint impingement.
 12. The paint spraygun of claim 6 in which said air nozzle comprises a housing having acylindrical chamber formed therein and an elongated exit slot and anentrance opening communicating with said chamber, a shiftable plugdisposed within said chamber, said shiftable plug including a pluralityof nozzle slots formed therein, each said nozzle slot being effective tolimit a fan of air having a different included angle, a central borecommunicating with said nozzle slots and an inlet opening communicatingwith said bore, said plug being shiftable to selectively align any ofsaid exit slots with the slot in said housing, and an electrode extendsinwardly through the entrance opening in said housing and the inletopening of said plug, said electrode being disposed in alignment withthe exit slot of said chamber and the nozzle slot of said plug inregistry therewith.
 13. A paint spray gun for use in a system includinga source of compressed air and a source of paint, said gun having abarrel portion, an air conduit through said barrel portion, means forconnecting said air conduit to said source of compressed air, a paintconduit through said barrel portion, means for connecting said paintconduit to said source of paint, a flat fan air nozzle in fluidconnection with said air conduit, a pair of flat fan-type paint nozzles,means supporting said air nozzle and said paint nozzles on said barrel,said air nozzle and said paint nozzles being in fluid communication withsaid air conduit and said paint conduit respectively, said air nozzlebeing effective to project a flat fan of air forwardly of said gun, saidpaint nozzles being effective to project flat fans of paint forwardly ofsaid gun which intersect said flat fan of air in a zone spaced forwardlyof said gun, means for ionizing said air in an area rearwardly of saidzone in which said air fan impinges upon said paint fans, whereby theair fan is effective to atomize said paint and to charge said paint toform a charged paint spray.
 14. The paint spray gun of claim 13 in whichthe paint nozzles are disposed so that the flat fans of paint intersectthe stream of air at an angle of from 30* to 70*.
 15. The paint spraygun of claim 14 in which the air and paint nozzles are disposed so thatthe air fan is disposed substantially on the bisector of the anglebetween said paint fans.
 16. The paint spray gun of claim 13 in whichthe air and paint nozzles are disposed so that the length of air travelof the air fan from the nozzle to the zone of air-paint impingement isfrom one-sixth to three-fourths the length of travel of the paint fansfrom the paint nozzles to the zone of air-paint impingement.
 17. Thepaint spray gun of claim 13 in which the air and paint nozzles aredisposed so that the air fan travels a distance of approximatelyone-eighth to three-eighths inch from the air nozzle to the point ofair-paint impingement.
 18. The paint spray gun of claim 13 in which theair and paint nozzles are disposed so that each paint fan travels fromapproximately three-eighths to three-fourths inch from the paint nozzleto the zone of air-paint impingement.
 19. The paint spray gun of claim13 in which said means for ionizing said air comprises an electrodedisposed within said air nozzle.
 20. The paint spray gun of claim 13 inwhich said air nozzle comprises a housing having a cylindrical chamberformed therein and an elongated exit slot and an entrance openingcommunicating with said chamber, a shiftable plug disposed within saidchamber, said shiftable plug including a plurality of nozzle slotsformed therein, each said nozzle slot being effective to limit a fan ofair having a different included angle, a central bore communicating withsaid nozzle slots and an inlet opening communicating with said bore,said plug being shiftable to selectively align any of said exit slotswith the slot in said housing, and an electrode extends inwardly throughthe entrance opening in said housing and the inlet opening of said plug,said electrode being disposed in alignment with the exit slot of saidchamber and the nozzle slot of said plug in registry therewith.